1. Field of the Invention
The present invention relates to a constant velocity universal joint for coupling a driving shaft and a driven shaft, to be used, for example, for a driving force-transmitting section of an automobile.
2. Description of the Related Art
A constant velocity universal joint has been hitherto used for a driving force-transmitting section of an automobile in order to transmit a rotary power or a torque of a driving shaft to respective axles through a driven shaft.
A constant velocity universal joint concerning the conventional technique is known, for example, as illustrated in FIG. 55, in which three track grooves 2 are formed along the axial direction on an inner surface of an outer wheel 1. Leg shafts 4, which protrude in the radial direction, are provided on a tripod member 3 which is arranged inside the outer wheel 1. A spherical roller 6 is fitted rotatably and slidably in the axial direction to an outer circumferential surface of each of the leg shafts 4 via a plurality of needle bearings 5. The spherical roller 6 is allowed to engage with a roller guide surface 7 disposed on both sides of the track groove 2.
However, in the case of the constant velocity universal joint concerning the conventional technique as described above, when a high load is applied in a state in which the tripod member 3 is inclined by a predetermined angle with respect to the axis of the outer wheel 1, then the force to press the roller guide surface 7 by the spherical roller 6, i.e., the surface pressure exerted on the roller guide surface 7 is increased, because each of the spherical rollers 6 linearly contacts with the roller guide surface 7 of the track groove 2. As a result, oil film breakage occurs at the contact surface between the spherical roller 6 and the roller guide surface 7, and lubrication failure occurs. Further, the following inconveniences take plate. That is, the lubrication failure causes formation of small holes on the surface of the spherical roller 6, i.e., so-called pits are generated. In other cases, irregularities are formed on the surface of the spherical roller 6, i.e., so-called adhesive wear occurs.
On the other hand, in the case of the constant velocity universal joint concerning the conventional technique as described above, when the tripod member 3 is inclined by a predetermined angle with respect to the axis of the outer wheel 1, a relationship is given as shown in FIG. 56, in which each of the spherical rollers 6 obliquely intersects the roller guide surface 7 of the cylindrical track groove 2 with each other. In such a state, it is impossible to allow the spherical roller 6 to perform proper rolling motion.
That is, the spherical roller 6 Intends to perform rolling motion in the direction indicated by an arrow A or an arrow B shown in FIG. 55, while the track groove 2 is cylindrical and it extends substantially in parallel to the axis of the outer wheel 1. Therefore, the spherical roller 6 is moved while being restricted by the track groove 2. As a result, a thrust force in the axial direction is induced by the slippage which is caused between the roller guide surface 7 of the track groove 2 and the spherical roller 6. The induced thrust force is increased in proportion to the increase of the angle of inclination of the tripod member 3 with respect to the outer wheel 1. It is feared to be difficult to smoothly transmit the rotary power of the driving shaft to the driven shaft. It is noted that a frictional resistance is generated by the reciprocating motion effected by the spherical roller 6 along the roller guide surface 7. In this context, the induced thrust force refers to a load resulting from the frictional resistance.
In order to solve the problem as described above, a constant velocity universal joint is known, which is disclosed, for example, in Japanese Laid-Open Patent Publication No. 3-168416. In this constant velocity universal joint, three ball grooves are formed in the axial direction at the inside of an outer wheel. Three pairs of balls are held in the respective ball grooves by the aid of holders respectively. A tripod member is incorporated into the inside of the outer wheel. Three leg shafts extending in the radial direction, which are arranged between the adjacent pairs of balls, are provided on the tripod member. Each of the leg shafts is formed with a spherical surface. A ball guide, which is formed with a spherical recess for engaging with the spherical surface, is provided between the spherical surface and the balls.
However, in the case of the constant velocity universal joint disclosed in Japanese Laid-Open Patent Publication No. 3-168416, when the operating angle between the first shaft provided at the closed end of the outer wheel and the second shaft provided on the tripod member is increased, there is a fear of occurrence of vibration in the direction of rotation and so-called beat sound resulting from backlash. The beat sound described above refers to a sound generated by looseness in the direction of rotation. Further, there is a fear that the following inconveniences may occur. That is, the balls tend to be disengaged from the holder upon assembly, it is difficult to retain the balls in the ball guide, a high technique is required for assembling, the assembling time is prolonged, and the operation efficiency is lowered.
A three-plane constant velocity joint is disclosed in Japanese Laid-Open Patent Publication No. 6-74243. In this case, an inner joint member is inserted into the inside of an outer joint member, and trunnions are provided on the inner joint member. Each of the trunnions is provided with a plurality of spherical balls. The spherical balls are constructed so that they are rollable along side walls which constitute a longitudinal chamber formed in the outer joint member. The spherical balls are held on the trunnion by the aid of a positioning spring installed to the trunnion.
However, also in the case of the three-plane constant velocity joint disclosed in Japanese Laid-Open Patent Publication No. 6-74243, when the operating angle between the outer joint member and the inner joint member is increased, there is a fear of occurrence of vibration in the direction of rotation and so-called beat sound resulting from backlash. Further, it is difficult to retain the spherical balls on the trunnion upon assembling. It is feared that the efficiency of the assembling operation is lowered.
Another constant velocity universal joint concerning the conventional technique is known, which is constructed, for example, as illustrated in FIG. 57. In this case, a pair of track surfaces 2a, 2b, which are opposed to one another and which have a circular arc-shaped cross-section, are formed in the axial direction on an inner wall surface of an outer joint member la. An inner joint member 3a, which is disposed between the pair of track surfaces 2a, 2b, is arranged in an internal hollow space of the outer joint member la. Further, a plurality of ball members 9 are provided rollably along the track surfaces 2a, 2b by the aid of a retainer 8 (see Japanese Patent Publication No. 7-74649).
However, in the case of the constant velocity universal joint concerning the conventional technique, when the spacing distance S between the pair of track surfaces 2a, 2b is smaller than a predetermined value, a large load is exerted on the ball members 9 which roll along the pair of track surfaces 2a, 2b. On the other hand, when the spacing distance S between the pair of track surfaces 2a, 2b is larger than a predetermined value, looseness occurs due to the gap between the ball members 9 and the pair of track surfaces 2a, 2b. As described above, the constant velocity universal joint concerning the conventional technique involves the inconvenience that the durability and the vibration characteristics are deteriorated by any dimensional error depending on the machining accuracy for the pair of track surfaces 2a, 2b which are formed on the inner wall surface of the outer joint member 1 in the mutually opposing manner.
Further, it is necessary that the pair of track surfaces 2a, 2b of the outer joint member 1 on which the ball members 9 roll have a hardness not subjected to plasticization caused by the contact surface pressure generated between the track surfaces 2a, 2b and the ball members 9. For this reason, it is necessary to apply a heat treatment to the pair of track surfaces 2a, 2b, which results in an inconvenience that the production cost becomes expensive.